# Common Formulas Sheet ```ETA CETa Exam Common Formulas
Ohm’s Law
Potential Divider
As the name says, we divide the potential or reduce the voltage in a circuit with help of potential divider.
E2
R
Vout = Vin * R2 / (R1 + R2)
ExI
Current Divider
It is used to redirect current flowing in a circuit.
E2
P
Iout = Iin * R1 / (R1 + R2)
2
I xR
P R
Balanced Wheatstone Bridge
A bridge used to measure resistances.
PxR
E
(R1 / R2) = (R3 / R4)
Voltage gain in decibels
Gain dB = 20 log (Vout / Vin)
Ratio of 2 power levels in decibels
Gain dB = 20 log (Vout / Vin)
Resonant frequency
FR = .159 / LC
P = I * E, the power being dissipated by the resistor is a product of the current and the applied voltage.
Resistors in series
R = R1 + R2 + R3...
Resistors in parallel
1 / R = (1 / R1) + (1 / R2) + (1 / R3)...
The resistance of a conductor at a temperature, t, is given
by the equation: Rt = R0(1 + αt + b t2 + y t3) where α, b, y
are constants and R0 is the resistance at 0&deg;C. Note that b &amp; y
are very small hence they can be neglected.
Therefore above equation simplifies to: Rt = R0(1 + αt) where
α = temperature coefficient of resistance.
Inductors connected in series
L = L1 + L2 + L3 +...
Inductors connected in parallel
1 / L = (1 / L1) + (1 / L2)...
Reactance of inductors
XL = 2 *  * f * L
where XL is reactance, f is frequency, and L is inductance
P
I
E
I
I
I x R P/R
P
I2
P
E
E
R
E = Voltage I = Current
P = Power R = Resistance
Current flowing in a Capacitor
The current flowing in a capacitor is proportional to the product of the capacitance, C, and the rate of change of applied
voltage.
i = C &times; (rate of change of voltage[d * V / d * t])
How to Compute Charge or Quantity of Electricity
Q = C* V
where Q is the charge (in coulombs), C is the capacitance (in
farads), and V is the potential difference (in volts).
Energy Storage in a Capacitor
W = &frac12; C * V2
where W is the energy (in Joules), C is the capacitance (in
Farads), and V is the potential difference (in Volts).
Capacitors connected in parallel
C = C1 + C2 + C3 +...
Capacitors connected in series
1 / C = (1 / C1) + (1 / C2)...
Reactance of capacitors
XC = 1 / (2 * * f * C)
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